An implantable cardioverter defibrillator (ICD) is an IMD that is capable of detecting a tachyarrhythmia (i.e. a fibrillation or abnormal high rate tachycardia) in a heart chamber and providing a high energy electrical shock into or across cardiac tissue to terminate the detected tachyarrhythmia. The ICD typically uses leads inserted into or positioned adjacent the heart chamber to sense electrical activity in the chamber. Cardiac depolarizations are sensed by sense amplifiers having inputs coupled to electrodes carried by the leads. The intervals between sensed depolarizations are measured and compared to threshold intervals to determine whether the chamber is in normal sinus rhythm, tachycardia, or fibrillation.
When detection criteria are met indicating tachycardia, the ICD may first attempt to terminate the tachycardia through anti-tachycardia pacing (ATP), which uses low energy pacing pulses. If ATP therapy is unsuccessful, then a high energy cardioversion shock, synchronized to sensed depolarization, is attempted. If fibrillation is detected, a high energy shock may be delivered without synchronization, and is referred to as a defibrillation shock.
To monitor ventricular tachyarrhythmia, the ICD senses ventricular depolarizations or -R-waves-. For accurate tachyarrhythmia detection, only one event (the R-wave) should be sensed during each normal sinus cardiac cycle. Oversensing of events other than the R-wave can result from sensing cardiac events such as ventricular repolarizations (T-waves) and far field R-waves, from double counting R-waves, and from sensing non-cardiac signals such as myopotentials from surrounding muscle tissue, noise from electromagnetic interference (EMI) external to the patient, or noise produced by a lead failure due to failed insulation, broken conductors or a poor connection to the ICD.
T-wave oversensing results when two ventricular sensed events occur during a cardiac cycle, one coinciding with the R-wave (depolarization) and the other coinciding with the T-wave (repolarization). T-wave oversensing can occur when there is a reduction in the R-wave amplitude (for example, due to a microdislodgement of the lead) or as a result of an increased T-wave amplitude (for example, due to a chemical/drug imbalance).
Oversensing T-waves by the ICD sense amplifiers can result in inappropriate shock therapies, which are painful to the patient and which waste energy and shorten the operating life of the ICD. In addition, timing either ATP or cardioversion to a T-wave that is incorrectly identified as an R-wave may induce an arrhythmia when one did not in fact exist.